1. Field of the Invention
The present invention relates to the reactivation, by ultrasound, of hydrogenation catalysts which are "deactivated", i.e. have lost appreciable effectiveness or have become ineffective, which catalysts contain ruthenium metal deposited on an aluminum oxide support.
2. Description of the Background
It is known that ruthenium deposited on aluminum oxide is a suitable catalyst for hydrogenation of unsaturated hydrocarbons and of aldehydes and ketones. As described Houben-Weyl, "Methoden der Organischen Chemie", Vol. 4/1c, p. 20, hydrogenation products obtained on Ru/Al.sub.2 O.sub.3 of cycloolefins multiply substituted on the double bond preferably have a cis-configuration, if the hydrogenation is carried out under certain temperature and pressure conditions. An example, of such as hydrogenation is the conversion of .alpha.-pinene to cis-pinane: ##STR1##
It is also known that the catalytic activity of Ru/Al.sub.2 O.sub.3 in practice drops relatively rapidly, e.g. in a fixed bed in a hydrogenation reactor through which an olefinic compound and hydrogen are passed, and this reduction in activity can lead to complete inactivation. The reduction in activity occurs even in the absence of so-called catalyst poisons, which, in general, are compounds containing heteroatoms with free electron pairs such as, e.g. Cl, Br, I, S, Se, Te, N, P, Sb, and As, which- compounds may be present in trace amounts in the substrate which is being hydrogenated, in solvents which may be used or in carrier gases or in gaseous hydrogen.
The usual measures to counter loss of hydrogenation activity of Ru/Al.sub.2 O.sub.3 catalysts consists of increasing the temperature and pressure and reducing the amount of substrate which is being hydrogenated, which substrate is passed over the fixed catalyst bed per unit time. However, the duration of effectiveness of these measures is limited. The loss of activity continues. Moreover, when the temperature is repeatedly raised, the selectivity of the Ru/Al.sub.2 O.sub.3 catalyst in forming the cis-hydrogenation product is lost.
The use of Ru/Al.sub.2 O.sub.3 catalysts for selective hydrogenation is thus limited under the state of the art. Ru/Al.sub.2 O.sub.3 catalysts, which have appreciably or completely lost their activity, do not have a different outward appearance; one cannot detect any caking together of catalyst granules which may occur as a result of deposition of higher molecular weight or polymerized byproducts on the surface.
Methods have been described for reactivating hydrogenation catalysts containing active components in the form of metals of Group VII of the Periodic System. According to U.S. Pat. No. 3,256,205, e.g., the catalyst is treated with acid solutions of pH .ltoreq.2. However, Ru/Al.sub.2 O.sub.3 catalysts cannot be treated in this manner because in acid medium .alpha.-pinene undergoes a Wagner-Meerwein rearrangement.
A need therefore continues to exist for a suitable method of reactivating deactivated Ru/Al.sub.2 O.sub.3 catalysts which does not have the known disadvantages.